The invention relates to anti-lock braking systems (ABS) for vehicles and more particularly to a wheel hub assembly having an exciter element incorporated therein.
Many specific types of ABS systems are known. Typically, these systems utilize a sensing device which continuously monitors wheel speed and produces a signal representative of the wheel speed. From the wheel speed, acceleration or deceleration of the wheel is determined and correlated to impending wheel lock and potential skid situations. If a skid situation is impending, the ABS will regulate braking fluid pressure so as to maintain maximum braking force without inducing wheel lock. Wheel speed sensors are also incorporated into traction control systems which reduce wheel spin for enhanced traction.
Known anti-lock braking systems utilize a sensor to detect the movement of an exciter ring. The exciter ring is typically annular and is provided with radially outward toothed projections. The ring is mounted onto a hub which rotates with the wheel of the vehicle. The sensor, often a variable reluctance magnetic pick-up, is mounted adjacent to the exciter ring on a non-rotating structure, such as the axle support or housing. As each tooth of the exciter ring passes the sensor, an electrical pulse is generated. These pulses are then analyzed by a logic unit to determine the acceleration or deceleration of the rotating wheel.
In order to ensure proper operation of the ABS, maintaining the proper spacing between the sensor and the exciter ring is crucial. If this spacing exceeds defined tolerances, the sensor will fail to read the passing teeth and in inaccurate readings and improper functioning of the ABS will result.
One prior method for attaching the exciter ring to the hub has been to press fit the exciter ring onto a machined surface of the hub. In this method, both the exterior diameter of the hub and the interior diameter of the exciter ring required precise machining to ensure proper spacing between the teeth and the sensor and to prevent tolerance stackups from becoming excessive. While press-fit wheel hub assemblies have been used extensively and have worked satisfactorily, they have exhibited some limitations. One such limitation is the potential for the exciter ring to loosen from the hub. Other limitations include cracking of the exciter ring and distortion of the ring or hub during press fitting.
To compensate for potential loosening of the exciter ring from the hub and to eliminate tolerance stack up, wheel hub assemblies having the exciter ring integrally cast with the hub have been developed. In these hub assemblies, the exciter ring is positioned within a mold and cast material for the hub is poured to embed a portion of the exciter ring into the hub itself. The embedded portion of the exciter ring prevents loosening of the ring from the hub.
U.S. Pat. application Ser. No. 682,098, filed Apr. 5, 1991, which is commonly owned by the assignee of the present application, discloses a steel exciter ring which is integrally cast into a iron hub. Because of the compatibility between the two ferrous materials, molecular bonding occurs to firmly secure the exciter ring and the hub together.
Where non-compatible materials are used, such as when the hub is formed from a non-ferrous material and the ring is formed from a ferrous material, provisions must be made to interlock the hub with exciter ring and prevent subsequent loosening. Typically, these provisions include lugs or other interlocking protrusions which extend interiorly of the hub. These lugs, however, increase the weight, complexity and cost of the rings themselves.
With the teeth extending radially outward from the exciter ring and above the surface of the hub, the potential exists for dirt, stones, mud, sand, etc. to collect between adjacent teeth. These deposits often disrupt proper operation of the ABS. To prevent "clogging" of the teeth, various type of seals, guards and shields have been developed. The guards adequately protect the teeth, however, they have further added to the bulk, weight and complexity of the wheel hub assemblies.
It is a principal object of the present invention to provide a wheel hub assembly for an ABS. The present invention, however, seeks to simplify construction while reducing both the weight and bulk of the wheel hub assembly.
Another object of this invention to provide a wheel hub assembly in which the spacing between the sensor and the sensed elements are readily maintained within acceptable tolerances, while preventing loosening of the sensed elements from the hub.
It is also an object of this invention to provide a wheel hub assembly in which rocks, stones, dirt, sand and other materials are prevented from becoming entrapped between the elements sensed by the sensor.
A further object of the invention is to provide for a composite wheel hub assembly which incorporates both ferrous and non-ferrous materials.
In achieving these objects, a method of constructing the present invention inserts a non-ferrous radially toothed ring into a casting mold and subsequently casts the hub, of a ferrous material, around the ring embedding the teeth within the raw casting. Gauging from the center of rotation of the hub assembly, the outside cylindrical surface of the raw casting, in the area of the hub barrel, is machined to the appropriate diameter. This machining removes a portion of the ferrous hub barrel and the non-ferrous ring leaving only a continuous, smooth cylindrical surface. The finished surface therefore includes portions of the ferrous hub and an exposed surface of the non-ferrous teeth. These teeth thus form non-ferrous, spaced apart "interruptions" in the ferrous material of the hub. The teeth, being embedded in the hub during casting and exposed during machining, remain "locked" in the hub as a result of the casting process. A sensor is located in close proximity to the "interrupted" exterior surface of the hub so that, during rotation of the wheel, the sensor will accurately detect the different material property characteristics of the hub assembly and allow wheel acceleration or deceleration to be determined.
By constructing the wheel hub assembly of the present invention with a continuous, smooth outer cylindrical surface, the sensed area is readily kept clean and free of foreign deposits, construction is simplified, size is reduced and weight is decreased. Problems such as cracking and distortion of the exciter ring, as previously occurred during press fitting of a ring onto the hub, are also eliminated.
Another advantage of the present invention is that the alternate materials sensed by the sensor form a smooth, coincidental cylindrical hub surface. Having been machined and gauged off of the actual center of rotation of the wheel hub assembly, tolerance stackups are eliminated and accurate spacing of the sensor, relative to the sensed area, is readily achieved. By being able to precisely locate the sensor in close proximity to the sensed surface, acceleration and deceleration, even during slow rotational speeds, can be accurately determined.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.